Historically, fungal infestations have caused significant losses to agricultural crops, and have been the cause of large scale famines and economic displacements. Fungal infections can cause pre-harvest damage to crops by killing them outright or weakening them so as to decrease yields and render the plants susceptible to other infections. Post-harvest, fungal infection also results in significant loss of agricultural products during storage, processing and handling.
Clearly, there is a significant need to control the fungal infection of agricultural products, and a number of chemical agents have been developed for this purpose, but to date no fully satisfactory agents have been found. Oftentimes, fungal control agents are highly toxic to crops and/or animals; consequently, restrictions are placed on their handling and use. Also, many presently available fungal control agents are of restricted utility; that is to say, a particular agent may be only effective against several types of fungus. As a result, a number of separate materials must often be employed in a particular agricultural setting. Also, a number of species of fungus have developed resistance to commonly employed fungicides.
Clearly, there is a need for a fungal control agent for plants which has broad activity against a variety of fungi, including those strains resistant to presently employed fungicides. The material should be of low toxicity to crops and to animals, stable in composition, easy to employ and preferably low in cost.
It is known that the cell walls of fungi are comprised of chitin, which is a natural, carbohydrate-based biopolymer. Chitin is an analog of cellulose in which the OH group at the C-2 position has been replaced by an acetamido group. Chitin is also found in a number of other natural sources, including the shells of arthropods. Chitin and chitosan are polymers having relatively high molecular weights, typically several hundred thousand or more; however, they can be hydrolyzed into lower molecular weight fractions, corresponding to shortened polymer chains. Previous research has suggested that chitin, or lower molecular weight fractions produced by its degradation, can in some instances, elicit antifungal responses in some plants; see for example M. G. Hahn et al. in Mechanisms of Plant Defense Responses; B. Fritig and M. Legrand, Kluwer Academic Publishers (Netherlands 1993, pages 99-116).
Chitosan is a semisynthetic derivative of chitin produced by the deacetylation of the nitrogen thereof so as to produce the ammonium salt. Chitosan has been shown to have some mild antifungal activity with regard to certain, particular fungal species, in some particular plants; see for example, L. A. 5 Hadwiger, J. M. Beckman; Plant Physiol. 66, 205-211 (1980); A. El Gharouth et al., Phytopathology, 84, 313-320 (1994); A. El Gharouth et al., Phytopathology, 82, 398-402 (1992); C. R. Allan, et al., Experimental Mycology 3:285-287 (1979); and P. Stossel et al., Phytopathology Z., 111:82-90 (1984). Specific hydrozylates of chitosan have also been described as having some antifungal activity. See for example, Kendra et al., Experimental Mycology 8:276-281 (1984). U.S. Pat. No. 5,374,627 describes the use of a composition of a high molecular weight chitosan hydrozylate (M.W. 10,000-50,000) and acetic acid for controlling fungus in certain crops. Japanese Patent Application 62-198604 describes the use of very low molecular weight chitosan hydrozylates (M.W..ltoreq.3,000) for the control of Alternaria alternata fungus in pears. It is further noted that this material is not effective, in pears, against other fungi such as botrytis.
U.S. patent application Ser. No. 08/453,651, of Ben-Shalom et al., discloses some specific oligosaccharide materials, including materials derived from chitin and chitosan, which have antifungal activity in plants. The disclosure of that application is incorporated herein by reference.
While some fungicidal activity of chitin and chitosan derived materials has been shown or suggested in the art, such utility is limited to particular pathogens and to particular plants. Furthermore, the prior art is uncertain as to which specific materials have beneficial effect. Certain references suggest that chitosan itself is to be employed as a fungal control agent, while other references suggest that high molecular weight fractions of chitosan are effective, and other references suggest that very low molecular weight fractions must be employed.
Another problem which the prior art has encountered in connection with chitin and chitosan based materials is that these materials have a very low solubility in water. Hence, solvent systems have to be employed to solubilize them, and this limits the utility of such materials. There is a need for a method whereby these materials can be rendered soluble in water having a neutral pH.
The present invention, as will be described in detail hereinbelow, is directed to fungal control agents derived from chitin and/or chitosan. This invention has identified a particular low molecular weight fraction of chitosan derived, material which is a broad spectrum antifungal agent, effective at very low doses. The present invention has also identified particular compositions containing this low molecular weight material and chitosan, in which the material and chitosan interact, synergistically, to provide an enhanced antifungal activity. The present invention has also identified particular low molecular weight oligomers of chitin which may be employed, either alone or with other materials, as agents for fungal control. It is to be understood that the terms "oligomers" and "low molecular weight material" are to a large extent synonymous in the context of this disclosure, since both refer to relatively short polymeric chains derived by reducing the molecular weight of chitin or chitosan. For the sake of clarity, the chitosan derived materials will generally be described as "low molecular weight" materials, while the chitin derived materials will be referred to as "oligomers." However, it is to be understood that either material could be considered an oligomer or a low molecular weight material. Collectively these substances will be deemed "materials of the present invention," or "antifungal materials."
The materials of the present invention are derived from natural sources, and have extremely low toxicity to animals and agricultural crops. In addition, the materials are stable, water soluble, easy to handle and low in cost. These and other advantages of the present invention will be readily apparent from the discussion, description and examples which follow.